Signal amplification is one of the most fundamental functions performed in analog, mixed-signal, and radio-frequency (RF) electronic systems. At lower signal frequencies, high gain amplifiers can be implemented with circuit topologies commonly employed in the design of operational amplifiers (opamps), and the power dissipated by such amplifiers can be low (especially if the required bandwidth is much less than 100 MHz). However, opamp-like circuits do not provide enough bandwidth for handling higher signal frequencies (e.g., above a few GHz), so different amplifier topologies (usually open-loop types) are employed in these frequency ranges.
One of the most common examples of a wideband, open-loop amplifier used for multi-GHz frequencies is the resistor-loaded differential amplifier. The amplifier stage comprises a differential input stage formed by differential transistor pair, load resistors and tail bias current sources. This differential amplifier topology is often referred to as a current-mode logic (CML) stage since it is the basic building block in that family of logic. The bandwidth of this type of amplifier is often limited by the RC time constant at the output nodes, which must be minimized for high frequency applications by increasing the bias current. As a result, the power dissipation of the amplifier can be undesirably high when a very large bandwidth (e.g., 10 GHz) is needed.
A more power-efficient alternative to the resistor-loaded differential amplifier is a current-integrating amplifier in which the load resistors are replaced by resettable capacitors at each output node and transistor switches that are driven by a clock signal in order to effect charging and discharging of the capacitors during a reset phase and an integrating phase. Integrating currents on parasitic capacitances is fundamentally much more power efficient than resistive current-to-voltage conversion. With typical circuit parameters, the power dissipation of a current-integrating amplifier may be almost an order of magnitude lower than that of a resistively loaded amplifier. Current-integrating amplifier circuits are commonly employed in analog signal summation circuitry to enable signal summation with high power efficiency made possible by the use of the current-integrating circuits.